One type of subsea wellhead assembly has a wellhead housing located at the upper end of the well. One or more strings of casing are supported by casing hangers in the wellhead housing. In a conventional subsea wellhead assembly, a tubing hanger secured to a string of tubing lands in the wellhead housing above the casing hanger. The tubing hanger typically has a plurality of vertical auxiliary passages that surround a vertical bore. The auxiliary passages provide penetration access through the tubing hanger for hydraulic and electrical components located downhole. Electrical and hydraulic lines extend alongside of the tubing to control downhole valves, temperature sensors and the like. The tubing hanger also has a tubing annulus passage that is offset and parallel to the bore for communicating with the tubing annulus.
A production tree lands on top of the wellhead housing. The production tree has a vertical bore that receives upward flow from the tubing hanger. The tree has valves and typically a choke for controlling flow from the well. The vertical auxiliary passages in the tubing hanger mate with auxiliary connectors protruding from the base of the tree. Auxiliary passages in the tree lead to a control unit that is mounted to the tree assembly.
One disadvantage of the conventional type of subsea wellhead assembly is that the tubing hanger must have a fairly large diameter because of the number of vertical passages extending through it. A large diameter tubing hanger requires a large diameter drilling riser through which the tubing hanger must be run prior installing the tree.
In another type of tree, often referred to as a “horizontal tree”, the tubing hanger lands in the tree after the tree has been installed, rather than landing in the wellhead housing. This tubing hanger has a vertical central bore, but does not have a tubing annulus passage extending through it. A lateral outlet in the tubing hanger extends from the central bore for production flow. Also, it normally does not have vertical auxiliary passages extending through it, thus may be smaller in diameter than a tubing hanger for a conventional tree. One type of horizontal tree tubing hanger has auxiliary passages that extend through the lower portion to a lower convex annular shoulder. The shoulder of the tubing hanger mates with a concave shoulder formed in the bore of the tree. Auxiliary passages extend through the tree and register with the auxiliary ports of the tubing hanger.